U.S. patent number 8,079,261 [Application Number 10/586,105] was granted by the patent office on 2011-12-20 for accelerometers.
This patent grant is currently assigned to Qinetiq Limited. Invention is credited to Roger Ian Crickmore, John David Hill, Peter James Thomas, John Peter Fairfax Wooler.
United States Patent |
8,079,261 |
Crickmore , et al. |
December 20, 2011 |
Accelerometers
Abstract
A fibre optic accelerometer particularly intended for use with
an interferometer using the compliant cylinder approach but further
providing a seismic mass at the core of the cylinder resulting in
improved sensitivity and rejection of out-of-axis inputs.
Inventors: |
Crickmore; Roger Ian
(Dorchester, GB), Hill; John David (Dorchester,
GB), Wooler; John Peter Fairfax (Dorchester,
GB), Thomas; Peter James (Hounslow, GB) |
Assignee: |
Qinetiq Limited
(GB)
|
Family
ID: |
31726357 |
Appl.
No.: |
10/586,105 |
Filed: |
January 12, 2005 |
PCT
Filed: |
January 12, 2005 |
PCT No.: |
PCT/GB2005/000078 |
371(c)(1),(2),(4) Date: |
July 14, 2006 |
PCT
Pub. No.: |
WO2005/068950 |
PCT
Pub. Date: |
July 28, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080229825 A1 |
Sep 25, 2008 |
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Foreign Application Priority Data
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Jan 17, 2004 [GB] |
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0401053.4 |
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Current U.S.
Class: |
73/514.26;
73/514.16 |
Current CPC
Class: |
G01P
15/093 (20130101); G01H 9/004 (20130101); G01V
1/181 (20130101); G02F 1/0134 (20130101) |
Current International
Class: |
G01P
15/13 (20060101) |
Field of
Search: |
;73/514.26,503,504.03,504.04,504.12,510,514.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2 386 687 |
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Sep 1993 |
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GB |
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2 208 711 |
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Apr 1999 |
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GB |
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10104056 |
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Apr 1998 |
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JP |
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WO 02/10774 |
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Feb 2002 |
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WO |
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WO 03/081186 |
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Oct 2003 |
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WO |
|
Primary Examiner: Williams; Hezron E
Assistant Examiner: Shah; Samir M
Attorney, Agent or Firm: McDonnell Boehnen Hulbert &
Berghoff LLP
Claims
The invention claimed is:
1. A fibre optic accelerometer comprising a seismic mass coaxially
constrained within a cylinder of compliant material, arranged to
prevent the cylinder deforming inwardly under axial compression,
the cylinder being circumferentially wound with optical fibre such
that axial compression of the cylinder by the seismic mass
increases stress in the optical fibre.
2. An accelerometer according to claim 1, wherein the seismic mass
includes a disc shaped portion.
3. An accelerometer according to claim 1, wherein the seismic mass
is secured by a tension member to a base plate.
4. An accelerometer according to claim 3, wherein a spacer is
provided between the cylinder and the base plate.
5. An accelerometer according to claim 4, wherein the spacer is
integral with the base plate.
6. An accelerometer according to claim 3, wherein the base plate is
integral with a platform or structure.
7. An accelerometer according to claim 1, wherein the optical fibre
is wound in a single layer.
8. An accelerometer according to claim 1 in which the seismic mass
is coaxially constrained within first and second cylinders of
compliant material, each cylinder being circumferentially wound
with optical fibre.
9. An accelerometer according to claim 8 in which the seismic mass
comprises a first circumferentially located bearer member arranged
to bear on an end of at least one of the compliant cylinders.
10. An accelerometer according to claim 9 in which the first
circumferentially located bearer member is arranged to bear on
respective ends of both of the compliant cylinders.
11. An accelerometer according to claim 9 comprising a second
circumferentially located bearer member arranged to bear on an end
of a second of the compliant cylinders.
12. An optical interferometer comprising an accelerometer according
to claim 1.
13. A fibre optic accelerometer according to claim 1, wherein said
compliant material is rubber or rubber like.
14. A method of measuring acceleration comprising providing a
seismic mass coaxially constrained within a cylinder of compliant
material, the cylinder being circumferentially wound with optical
fibre, axially displacing the seismic mass so as to compress the
cylinder thereby increasing the stress induced in the optical
fibre, and determining a stress value in the optical fibre.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to accelerometers and particularly
fibre optic accelerometers for use in interferometers.
(2) Description of the Art
The need to monitor extremely low levels of vibration in areas such
as security, seismic survey and condition monitoring of machinery
and such like has spurred the development of ever more sensitive
accelerometers. Fibre optic technology has been applied to this
particular field in the form of fibre-optic accelerometers based on
interferometric techniques. The compliant cylinder approach to the
design of a fibre-optic accelerometer is particularly effective
when incorporated in such an interferometer. In one known approach
a seismic mass is held in place by two compliant cylinders and
around the circumference of each cylinder there being wound a
single mode optical fibre, which form the arms of an
interferometer. In another approach, a single compliant cylinder 2
loaded with a seismic mass 4 as shown in FIG. 1 is wound
circumferentially with an optical fibre 6.
Whilst the abovementioned approaches have found acceptance, there
remains a need to increase yet further the sensitivity of the
accelerometer beyond that currently achievable and in particular to
do so without any increase in component size. The present invention
seeks to improve the sensitivity of a fibre wound compliant
cylinder accelerometer whilst simultaneously seeking to avoid
additional cost and complexity of construction.
SUMMARY OF THE INVENTION
Thus, according to one aspect of the invention, there is provided a
fibre optic accelerometer comprising a seismic mass coaxially
constrained within a cylinder of compliant material, the cylinder
being circumferentially wound with optical fibre.
Preferably, the accelerometer is mounted on a plate which may or
may not in practice be an integral part of a platform or structure
on which the accelerometer is deployed. Conveniently, a tension
member retains the accelerometer against the plate. The tension
member may be a bolt or other well known tensioning component.
Equally, the tension member may be provided by an enclosure or can
acting on the accelerometer. Advantageously, the tension member
acts on the accelerometer via a compliant material washer whilst a
rigid support ring is interposed between the plate and the cylinder
to ensure that relative movement is possible.
It will be recognised that a suitable compliant material for the
cylinder will have a relatively low Young's modulus but with a
Poisson's ratio close to 0.5, such that the stiffness of the
accelerometer arises more from the circumferential winding than the
cylinder itself. Thus for a particular force acting on the
cylinder, the greater the strain induced in the fibre and hence
sensitivity of the accelerometer. Furthermore, by constraining the
seismic mass coaxially within the cylinder, the tendency present in
prior art devices for the cylinder to buckle or otherwise respond
unfavourably to acceleration orthogonal to the cylinder axis is
limited. Advantageously, this leads to improved performance of
devices incorporating the accelerometer in which sensitivity in a
single axis is paramount.
It will be further recognised that by reducing the wall thickness
of the cylinder the sensitivity of the accelerometer can be still
further increased. Prior art devices have hitherto sought to
increase sensitivity either by increasing the seismic mass and/or
the height of the cylinder supporting the seismic mass. Both
approaches whilst increasing the desired sensitivity may also have
the problem of increased sensitivity to orthogonal acceleration
mentioned above and will result in an increased accelerometer size.
With the trend towards miniaturisation of components, the present
invention lends itself to providing improved performance to prior
art devices for a given volume and mass.
The invention is also directed to methods by which the described
apparatus operates and including method steps for carrying out
every function of the apparatus.
In particular according to a further aspect of the present
invention then is provided a method of measuring acceleration
comprising providing a seismic mass coaxially constrained within a
cylinder of compliant material, the cylinder being
circumferentially wound with optical fibre, axial displacement of
the seismic mass deforming the cylinder so as to vary the stress
induced in the optical fibre.
There is also provided a method of measuring acceleration
comprising providing a seismic mass coaxially constrained within
first and second cylinders of compliant material, each cylinder
being circumferentially wound with optical fibre, axial
displacement of the seismic mass deforming each cylinder so as to
vary the stress induced in respective optical fibres.
The preferred features may be combined as appropriate, as would be
apparent to a skilled person, and may be combined with any of the
aspects of the invention.
DESCRIPTION OF THE FIGURES
In order to assist in understanding the invention, a particular
embodiment thereof will now be described, by way of example and
with reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional side view of a prior art fibre optic
accelerometer;
FIG. 2 is a cross-sectional side view of a first fibre optic
accelerometer in accordance with the present invention;
FIG. 3 is a schematic view of an optical interferometer
incorporating an accelerometer of FIG. 2; and
FIG. 4 is a cross-sectional side view of a second fibre optic
accelerometer in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 2, the fibre optic accelerometer 1 is mounted
on a base plate 3 via a rigid support ring 5. The ring 5 can be
formed either as a relief in the base plate 3 or perhaps more
conveniently, it can be provided as a separate component, thereby
allowing differing sizes of accelerometer 1 to be mounted on the
base plate 3. The base plate 3 itself is produced from a rigid
material, typically steel although other metals and composites may
suggest themselves to those skilled in the art. Furthermore, it
should be understood that references throughout the description to
a base plate are also intended to encompass the direct mounting of
the accelerometer to a platform or other structure.
The support ring 5 is in contact with a first end face of a
compliant cylindrical member 7. The cylindrical member has
relatively thin wall 9 and a coaxial void 11 such that a seismic
mass 13 may be received therein. The compliant cylindrical member 7
is formed from a material having a relatively low Young's modulus
such that it is capable of deformation under low levels of loading
in an axial direction. Typically, a rubber or rubber like material
may be utilised. Such materials also have a Poisson ratio
approaching a maximum of 0.5 meaning that an efficient transfer of
axial stress into circumferential stress in the cylinder 7 can take
place. Ideally the inner surface of the cylinder and outer surface
of the seismic mass are shaped so as to prevent the cylinder
deforming inwardly under axial compression of the cylinder.
The seismic mass 13 is held by a tension member in the form of a
bolt 15 secured to the base plate 3. Whilst in a non-illustrated
embodiment the tension member is provided by an enclosure or can,
other forms of tension member will be readily apparent to those
skilled in the art. The bolt 15 bears on the seismic mass 13 via an
elastomeric member which is most easily provided by a pad 17 of
rubber of rubber-like material. The seismic mass 13 itself Is so
shaped that a generally disc shaped portion 19 bears on a second
end face 21 of the compliant cylindrical member 7. In use,
acceleration forces acting on the seismic mass 13 bring about a
displacement which is coupled to the cylindrical member 7. Without
the tension member 15, there would be no coupling of displacement
to the cylindrical member 7 where the sense of acceleration is such
as to urge the disc shaped portion 19 out of contact with the
second end face 21. In effect, the tension member 15 preloads the
cylindrical member 7 with an initial displacement. Depending on the
range of acceleration expected, the preload may be varied by
altering the level of tension provided by the tension member
15.
The cylindrical member 7 is wound with a length of optical fibre
23. The winding may be single or multi layered The optical fibre 23
is wound about an external surface 25 of the cylinder 7 and may be
secured mechanically, adhesively or through another or combination
of techniques to ensure that as completely as possible the
possibility of slippage between the fibre 23 and the cylinder
surface 25 is minimised.
It will be appreciated that the optical fibre 23 constrains the
cylindrical member 7 against circumferential deformation thus
generating a level of hoop stress in the fibre 23. This hoop stress
alters the physical characteristics of the optical fibre 23 such
that by incorporating the accelerometer in one arm of an optical
interferometer (FIG. 3) a stress value proportional to the
acceleration acting on the accelerometer 1 can be determined.
In this arrangement compression of the compliant cylinder by
displacement of the seismic mass effectively increases the stress
in the optical fibre; conversely expansion of the compliant
cylinder decreases stress in the optical fibre.
FIG. 3 shows the accelerometer 1 as an element in an optical
interferometer 30 used to determine acceleration. In this
embodiment there is provided a source of laser light 31 a coupler
32, coupling two arms 33,34 of fibre optic cable and an output to a
display 35. One of the arms 33 contains the accelerometer 1 whilst
the other arm 34 includes a polarisation corrector 36. The
operation of such an interferometer 30 will be apparent to those
skilled in that art just as those skilled in the art will recognise
that this interferometer is purely Illustrative and that the
accelerometer of the invention may be deployed in a host of
interferometer applications.
Whilst those skilled in the art will recognise the improvements in
resistance to off-axis acceleration effects conferred by the above
described embodiment, further steps may be taken to minimise the
detrimental effect of such inputs. Accordingly, a shim may be added
between the tension member and the seismic mass to resist out of
axis inputs whilst maintaining on-axis sensitivity.
Referring now to FIG. 4, in a further embodiment the seismic mass
11 is located coaxially inside two separate cylinders 2, 6 of
compliant material. Each cylinder is surrounded by a separate
length of optical fibre 4 and 7. The end faces of the two compliant
cylinders nearest the centre of the sensor each rest on a bearer
member (in this case in the form of a circumferential protrusion
from the mass) extending outwardly from the seismic mass 19. Whilst
in the embodiment shown a single bearer member is shown bearing on
one end of each cylinder, clearly two separate bearer members may
be employed to bear on each of the respective complaint cylinders.
Nor is it essential that the bearer member be strictly uniform in
form around the circumference of the mass, merely that it
adequately transfers the effects of the axial displacement of the
mass to each of the complaint cylinders. The opposite end of each
compliant cylinder is in contact with separate support rings (or
more generally end support members) 5 which are themselves attached
to two end plates 3, one of which is situated at either end of the
accelerometer. A tensioning device 15 is used to pull the two end
plates together so that when the accelerometer is stationary both
of the compliant cylinders are in a state of compression. In this
diagram the tensioning device is shown in the form of a bolt 15
but, as explained previously, it may also take other forms.
As will be recognised by those skilled in the art, acceleration in
one axial direction will increase the axial compression in one
compliant cylinder and decrease it in the other, and so the effects
induced in the two fibre coils 4,7 will be equal in magnitude but
opposite in sense. As again will be recognised by those skilled in
the art if the two fibre coils are used in two different arms of an
interferometer, 33 and 34 in FIG. 3, the changes in the two coils
will add together and so the sensitivity of the accelerometer will
be twice as a large as if a single coil were used in one arm of the
interferometer. This embodiment also has the advantage that if the
accelerometer experiences an acceleration orthogonally to the axis,
any signals induced in the two fibre coils will tend to cancel out
if they are used in separate arms of an interferometer.
As in the earlier embodiment, in this arrangement compression of a
compliant cylinder by displacement of the seismic mass effectively
increases the stress in the optical fibre wound around the
cylinder; conversely reducing the compression applied to a
compliant cylinder decreases stress in the optical fibre. It is
noted that whilst it may be convenient for the seismic mass and
surrounding compliant cylinder to have substantially circular
cross-sections, this is not essential for the operation of the
apparatus and other cross-sections are equally possible including,
for example, oval.
Any range or device value given may be extended or altered without
losing the effect sought, as will be apparent to the skilled person
for an understanding of the teachings herein.
* * * * *